황민욱 (POSTECH)

서종철 (POSTECH)

황민욱 (POSTECH)

김민수 (POSTECH)

남경민 (POSTECH)

김경환 (POSTECH)

서종철 (POSTECH)

Charged nanodroplets generated by electrospray ionization can exhibit molecular behaviors distinct from those in bulk solution. Their size and morphology are important because they influence molecular confinement, ion distributions, and local electrostatic and solvation environments. However, direct characterization remains challenging because nanodroplets are extremely small and undergo rapid motion, evaporation, surface fluctuations, and Coulombic deformation. Here, we investigated the size and morphology of charged nanodroplets generated by vacuum electrospray using small-angle X-ray scattering with an X-ray free-electron laser (XFEL-SAXS). A custom vacuum electrospray chamber was developed to introduce the droplets directly into the XFEL interaction region. The ultrashort and intense X-ray pulses enabled single-shot measurements of rapidly moving droplets, allowing individual droplet sizes and their statistical distribution to be determined.

The droplet-size distribution showed a bimodal profile. The characteristic droplet sizes varied systematically with the measured liquid flow rate and followed established electrospray scaling laws, indicating that the initial droplet size was mainly controlled by the flow conditions at the emitter. Most SAXS patterns were consistent with spherical droplets. Several non-spherical patterns could be reproduced using geometrical models resembling charged-droplet deformation and fission morphologies previously observed at the micrometer scale. These patterns were tentatively assigned to ellipsoidal, conical, double-cone, and double-droplet structures. These results demonstrate that XFEL-SAXS can provide both statistical size distributions and morphological snapshots of highly dynamic charged nanodroplets.